Biomedical Engineering Reference
In-Depth Information
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Austenite
Martensite
Austenite Martensite
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Strain
6.7
Schematic representation of stress induced martensite
transformation at constant temperature.
extend from bone anchors to a variety of devices for osteosynthesis bone
staples.
The potential release of ni has been the driving force for great activity
in its surface modification. Thermal oxidation of the alloy, performed under
low oxygen partial pressure, leads to the formation of a pure stochiometric
Tio
2
on the surface.
8
This layer improves the electrochemical corrosion
resistance of the alloy by increasing the breakdown potential and decreasing
the maximum current density and ni ion release and, therefore, may avoid
toxic reactions associated with ni.
9
a critical overview of nitinol surfaces and
their modifications for medical applications has been recently published.
10
6.3.2 Zirconium alloys
zr-2.5% nb is a biocompatible, high-strength alloy that possesses an elastic
modulus of 100 GPa. Thermal oxidation of the alloy under dry oxygen
flow at temperatures near the eutectoid temperature (590-700ºC) for a few
hours gives rise to the formation of an outer zirconia layer of about 5 mm
in thickness. The resulting monoclinic zirconia surface contains grains that
are 40 nm wide and 200 nm long, arranged in a brick work pattern that is
resistant to grain pull out and lateral fracture.
11
The oxidized alloy material,
which combines the abrasion resistance of ceramics with the toughness
of metals, was recently introduced commercially for knee arthroplasty
components.
12
The thermally oxidized alloy is burnished to create a smooth
bearing surface. In this condition the material shows excellent wear behaviour
against polyethylene components, with reduced wear particle generation.
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